Three Anionic IndiumâOrganic Frameworks for Highly Efficient and

Article Cite This: Inorg. Chem. 2019, 58, 8396−8407

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Three Anionic Indium−Organic Frameworks for Highly Efficient and Selective Dye Adsorption, Lanthanide Adsorption, and Luminescence Regulation Xiangjing Gao, Guohao Sun, Fayuan Ge, and Hegen Zheng* State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, P. R. China Downloaded via UNIV OF SOUTHERN INDIANA on July 21, 2019 at 10:13:14 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Through solvothermal reaction of InCl3 and tetracarboxylate ligands with different substituent groups on diphenyl ethers, three new anionic indium−organic frameworks have been successfully prepared. They are {[(CH3)2NH2]In(G1)(H2O)}·9DMF (1), {[(CH3)2NH2]In(G-2)}·15DMF (2), and {[(CH3)2NH2]2In2(G-3)2}·16DMF (3) {DMF = N,N′dimethylformamide; H4(G-1) = 5′,5″″-oxybis(2′-methoxy[1,1′:3′,1″-terphenyl]-4,4″-dicarboxylic acid); H4(G-2) = 5′,5″″oxybis(2′-amino[1,1′:3′,1″-terphenyl]-4,4″-dicarboxylic acid); H4(G-3) = 5′,5″″-oxybis([1,1′:3′,1″-terphenyl]-4,4″-dicarboxylic acid)}. Compounds 1−3 can be simplified as unimodal 4-connected frameworks with different topological types: lon, cag, and dia, respectively. Compounds 1 and 3 display 2-fold interpenetrating nets, while compound 2 is non-interpenetrating. Compounds 1 and 3 can adsorb cationic methylene blue (MB) with good capacity and a high adsorption rate due to their anionic frameworks and channel-type voids. In particular, compound 1 exhibits great selectivity for cationic MB in the mixtures of MB and methyl orange. In addition, the adsorption behavior of rare earth ions (Eu3+ and Tb3+) on compounds 1 and 3 has also been studied. Due to the different structural features and channel sizes of compounds 1 and 3 caused by different substituents on the ligands, the adsorption properties of rare earth ions on the two compounds are different.

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INTRODUCTION In recent years, organic dyes have been widely used in the textile industry, printing industry, and other related industries.1 A large amount of wastewater that contains various types of dyes is discharged, which becomes a serious environmental problem as it causes a series of negative effects on humans and animals.2 Currently, there are many methods for removing organic dyes from industrial wastewater such as chemical degradation, biodegradation, and physical adsorption.3−10 However, these methods usually suffer from high costs and low efficiencies. Usually, the adsorbents in these methods cannot be recycled, which can cause secondary pollution. More importantly, the poor selectivity of these adsorbents limits their application in the dye separation and purification process, which requires the adsorbents to have high selectivity to achieve targeted dyes with high purities. In addition, rare earth ions are important strategic resources, and their separation and purification have great significance. Additionally, choosing suitable adsorbents for rare earth ions is a feasible approach for obtaining tunable luminescent materials. Therefore, it is © 2019 American Chemical Society

necessary to explore highly selective adsorbents for dyes and rare earth ions. As an adsorbent, metal−organic frameworks (MOFs) have many advantages such as an adjustable pore structure and a large specific surface area. They are widely used in gas adsorption and separation and recognition of metal ions and small organic molecules.11−15 MOFs can be classified into two categories, ionic16−24 and neutral frameworks. Compared with the latter, the ionic frameworks are particularly suitable as the materials for adsorbing charged dyes or metal ions. However, the study of ionic frameworks is much rarer than the study of neutral ones. In-based MOFs are mostly ionic frameworks because indium is mainly present in its trivalent state. A large number of In-based ionic frameworks have emerged since Zhao et al.25 reported the first In-based ionic framework. Intensive studies of ionic In-MOF adsorbents revealed that ionic In-MOFs could aid in the selective absorption and Received: February 21, 2019 Published: June 10, 2019 8396

DOI: 10.1021/acs.inorgchem.9b00499 Inorg. Chem. 2019, 58, 8396−8407

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Inorganic Chemistry Scheme 1. Three New Tetracarboxylate Ligands with Different Substituents Based on Diphenyl Ethers

Figure 1. (a) ORTEP drawing of 1 (hydrogen atoms omitted for the sake of clarity). (b) Perspective of a single framework of 1 along the a axis. (c) Perspective of the final 2-fold interpenetrating 3D frameworks with channels along the a axis. (d) Schematic representation of the lon topology structure.

separation of dye molecules.27−31 They can also be used to adsorb and recognize rare earth ions.26−28 However, studies of the selective adsorption on anionic In-MOFs in multicomponent aqueous systems remain elusive. By using the InCl3 and tetracarboxylate ligands with different substituent groups (Scheme 1) on diphenyl ethers, three novel anionic indium−organic frameworks have been successfully prepared. The selective adsorption on those In-MOFs in a multicomponent aqueous system has been studied. Interestingly, 1 displays great selectivity for adsorbing cationic methylene blue (MB) in the mixtures of MB and methyl

orange (MO) in an aqueous solution. In addition, 3 shows great selectivity for adsorbing Eu3+ in the mixtures of Eu3+ and Tb3+ in an aqueous solution. These phenomena can be explained by the charge properties and channel structures of the frameworks.

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RESULTS AND DISCUSSION Synthesis and Crystal Description. The solvothermal reaction of InCl3 and three different tetracarboxylate ligands in N,N′-dimethylformamide (DMF) yielded three different crystals. Their formulas, which can be determined by single8397


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Figure 2. (a) ORTEP drawing of 2 (hydrogen atoms omitted for the sake of clarity). (b) Perspective of the framework of 2 along the c axis. (c) Perspective of the largest 1D channels along the c axis. (d) Schematic representation of the cag topology structure.

coordination modes: bidentate chelate coordination mode and monodentate coordination mode, which is not present in the other two compounds. The In−O bond lengths range from 2.029 to 2.387 Å, which are comparable to those reported in the literature.33−37 The [In(OH2)(O2C−)4] secondary building units (SBUs) are connected by the (G-1)4− linkers, resulting in a 2-fold interpenetrating three-dimensional (3D) framework. As shown in Figure 1c, there are spindly one-dimensional (1D) channels of 6.25 Å × 16.16 Å (considering van der Waals radii) along the a axes. Each In(III) can be simplified to a 4connected node by using a topological approach. The (G-1)4− ligands can also be simplified to a 4-connected node. Therefore, the whole structure can be regarded as a lon topology (Figure 1d) that has a point symbol of {66}. The 2fold interpenetration can be classified as type IIa, Z = 2[1 × 2] (Zt = 1; Zn = 2). The solvent-accessible free volume of compound 1 is ∼48.1%, which is calculated by the SOLV program of PLATON. Compound 2 crystallizes in orthorhombic space group Pbca. One independent In(III), one (G-2)4− ligand, one dimethylamine cation, and lattice DMF molecules are in the minimum asymmetric unit. As shown in Figure 2a, eight oxygen atoms from the (G-2)4− ligands coordinated to In(III) form a distorted triangle dodecahedron. The In−O bond lengths range from 2.241 to 2.339 Å. All of the carboxylic groups of (G-2)4− ligands manifest bidentate chelate coordination mode

crystal X-ray diffraction (SCXRD) and elemental analysis, were {[(CH3)2NH2]In(G-1)(H2O)}·9DMF (1), {[(CH3)2NH2]In(G-2)}·15DMF (2), and {[(CH3)2NH2]2In2(G-3)2}·16DMF (3). As shown in Figure S3, the experimental powder X-ray diffraction (PXRD) patterns and the simulated PXRD patterns are similar, which indicate the compounds are pure-phase. The macroscopic morphology of these three compounds is different. Compounds 1 and 3 are colorless and transparent. Compound 1 is lamellar. Compound 3 is dendritic. Compound 2 is blocky. Crystal Structure Description. As revealed by SCXRD, the crystal structures of these three compounds are also very different. The differences in the crystal structures can be related to the different substituents (the dimethylamine cation generated by decomposition of DMF and other lattice solvent molecules are squeezed by the PLATON program because of severe disorder in these three compounds).32 Compound 1 crystallizes in the monoclinic crystal system and C2/c space group. In the asymmetric unit of 1 are one independent In(III), one (G-1)4− ligand, one coordinated water, one dimethylamine cation that serves as the counterion, and lattice DMF molecules. As shown in Figure 1a, In1 is coordinated by six oxygen atoms from the (G-1)4− ligands and one oxygen atom from the coordinated water. The In−O polyhedron forms a distorted pentagonal bipyramid. Interestingly, the carboxylic groups of (G-1)4− ligands manifest two 8398


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Figure 3. (a) ORTEP drawing of 3 (hydrogen atoms omitted for the sake of clarity). (b) Perspective of a single framework of 3 along the c axis. (c) Perspective of the final 2-fold interpenetrating 3D frameworks with channels along the c axis. (d) Schematic representation of the dia topology structure.

Figure 4. Photographs taken during the adsorption on 1 show the change in color of the MB solutions.

that forms the [In(O2C−)4] SBUs. The (G-2)4− linkers interconnect those SBUs to form a 3D framework. Compound 2 has irregular and accessible channels in all directions. The largest one is the rhombic channel along the c axes (Figure 2c), which is ∼10.5 Å × ∼10.6 Å (considering van der Waals radii). Using a topological approach, each In(III) and (G-1)4− ligand can be simplified to a 4-connected node. The whole structure can be identified as a cag topological type (Figure 2d) with a point symbol of {4·65}. The solvent-accessible free volume of compound 2 accounts for 82% of the total unit cell volume. However, when the solvent is removed, the blocky crystals are crushed into a powder. Compound 3 crystallizes in the monoclinic crystal system and P21/c space group. In the asymmetric unit of 3 are two

independent In(III) atoms, two (G-3)4− ligands, two dimethylamine cations, and lattice DMF molecules (Figure 3a). In1 and In2 in this structure have the same coordination number and geometry as In(III) in compound 2. The carboxylic groups of (G-3)4− ligands in 3 also have the same coordination mode with compound 2. The In−O bond lengths range from 2.183 to 2.358 Å. The [In(O2C−)4] SBUs are connected by the (G3)4− linkers resulting in a 2-fold interpenetrating 3D framework. There are irregular and narrow channels along the a and b axes. Along the c axes, there are two types of accessible 1D channels (Figure 3c): relatively well-shaped rhombic and irregular channels of 9.35 Å × 9.35 Å and 7.80 Å × 9.87 Å, respectively. Each In(III) and (G-3)4− ligand can be simplified to a 4-connected node. The whole structure can be 8399


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Figure 5. Sequential UV−vis spectra of (a) MB (11.18 mg/L) and (b) MO (100 mg/L) in an aqueous solution after the addition of compound 1. The inset shows a close-up of the −0.005 to 0.1 au region of panel a.

Figure 6. (a) Adsorption kinetics of MB (11.18 mg/L) on compound 1 at ambient temperature. (b) Kinetic data fitting of MB (11.18 mg/L) adsorption on 1 using the pseudo-second-order kinetic model (R2 = 0.999).

regarded as a dia topology (Figure 3d) that has a point symbol of {66}. The 2-fold interpenetration can be classified as type IIa, Z = 2[1 × 2] (Zt = 1; Zn = 2). The solvent-accessible free volume of compound 3 is ∼63.4%. Physical Adsorption of Dyes. Considering the channeltype void spaces and the charged frameworks in compounds 1 and 3, we chose the linear and charged dyes MB and MO as the subjects of our study (compound 2 was excluded because of its instability). Compounds 1 and 3 can remain stable after immersion in H2O and adsorbing MB (Figure S4a,b). The concentration of the solution of the dyes is determined by the calibration curves for MB and MO, which are shown in Figure S5. Single-Dye Adsorption in an Aqueous Solution on Compounds 1 and 3. As-synthesized compound 1 was dispersed in a freshly prepared aqueous solution of 11.18 mg/L cationic MB. After 15 mg of 1 was added to 60 mL of a MB solution with gentle magnetic stirring at ambient temperature, the color of the MB solution dramatically changed from blue to colorless (Figure 4). This phenomenon was also monitored by ultraviolet−visible (UV−vis) absorption spectroscopy. The maximum absorption peak of MB at 664 nm had a dramatic intensity drop after the mixture had been stirred for only 5 min as shown in Figure 5a. Moreover, the peak almost disappeared after 60 min, which indicates that subtotal MB molecules were adsorbed by 1. However, after the mixture had been stirred for 240 min under the same conditions except using a higher MO concentration of 100 mg/L, there was a negligible adsorption change for MO, which has the maximum absorption peak at 465 nm. An only 3.00% decrease was observed in the UV−vis absorption spectra (Figure 5b). Compound 1 shows a higher selectivity for cationic MB than for anionic MO, which is dependent on the negative charge of the framework.

Moreover, the adsorption kinetics of MB molecules on 1 show that the adsorption is not only efficient but also very fast; 68.3% of total MB was adsorbed within only 5 min. A pseudosecond-order kinetic model can be fit to the adsorption kinetics of MB molecules on 1 (Figure 6). Second-order kinetic constant k2 (t/qt = 1/k2qe2 + t/qe), which is related to the adsorption rate, is 0.014 g mg−1 min−1 (R2 = 0.999). It is a superior high value when compared with those of other MOFs used for MB adsorption in an aqueous solution.30,38−41 Similar results can be obtained by increasing the initial concentration of MB to 95.64 mg/L (Figure S6). Compound 1 can adsorb 72.3% of the MB in 5 min and nearly 100% in 60 min with gentle magnetic stirring. These results demonstrate that 1 can efficiently and rapidly adsorb MB over a wide MB concentration range. Even without magnetic stirring, 93.8% MB can be adsorbed on 1 in 180 min (Figures S7 and S8). Thus, compound 1 shows good application prospects in adsorbing cationic organic dyes from wastewater. In addition, the maximal adsorption capacity of dyes on 1 has also been tested. As-synthesized compound 1 was dispersed in freshly formulated aqueous solutions with different concentrations of MB. The concentrations of MB after adsorption were obtained by UV−vis spectroscopy after several days. As shown in Figure 7, compound 1 exhibits a type I adsorption isotherm and the maximal adsorption capacity Q0 derived from the Langmuir equation is ∼497.51 mg/g, which is relatively high among the values of reported materials.40,42−44 Fifteen milligrams of as-synthesized compound 3 was dispersed in a freshly formulated 50 mL aqueous solution of 13.09 mg/L cationic MB with gentle magnetic stirring at ambient temperature. Compared with the adsorption of MB on 1, which showed a rapid change in color, the MB solution gradually changed from blue to colorless at a much slower rate during this experiment (Figure 8). According to the UV−vis 8400


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for the adsorption of the mixtures of MB and MO in an aqueous solution. Indeed, compound 1 shows a good selectivity for MB, although the concentration of MO is 4 times higher than the concentration of MB. The color of the mixture changed quickly from green to yellow within 60 min (Figure 12). As shown in Figure 13, compound 1 exhibits a high MB adsorption capacity, which is 92.40% after 60 min and 97.75% after 240 min, with a negligible MO adsorption capacity. Single-Rare Earth Ion Adsorption on Compounds 1 and 3 in an Aqueous Solution. The aqueous solutions of Eu(NO3)3·6H2O and Tb(NO3)3·6H2O at different concentrations were prepared by adding ground compounds 1 and 3 to the solutions separately. The suspension was centrifuged and washed with distilled water three times before the fluorescence of the suspension was measured. Compounds 1 and 3 show different adsorption characters of different rare earth ions. The infrared (IR) spectra (Figure S2) and the PXRD patterns (Figure S4c,d) of the Ln3+@In-MOFs confirmed that the In-MOF maintained its crystalline integrity and original coordination mode after lanthanide adsorption. As the initial concentration of the rare earth ion [Eu(NO3)3· 6H2O or Tb(NO3)3·6H2O] increases, the intensities of the characteristic fluorescence emission peaks increase nonlinearly (as shown in Figure 14). When plotted against the initial concentration, the luminescence intensity ratios of I (Eu3+, 616 nm) to I (1, 421 nm) and I (Tb3+, 545 nm) to I (1, 421 nm), which are related to adsorption per unit quality on 1, increase significantly and remain constant thereafter (Figure 14b,d). The intensity ratio of I (Eu3+, 616 nm) to I (1, 421 nm) reaches a maximum value of ∼2.88, whereas the intensity ratio of I (Tb3+, 545 nm) to I (1, 421 nm) has a maximum value of ∼2.25. These observations indicate that 1 shows similar Eu3+ and Tb3+ adsorption capacities. However, the adsorption capacity of 1 for Tb3+ is slightly smaller than that for Eu3+, which may be due to the larger hydrated Tb3+ ion radius compared to that of hydrated Eu3+. The CIE coordinates show that the luminescence color of 1 changes from blue to purplish pink when adsorbing Eu3+ and changes from blue to bluish green when adsorbing Tb3+ (Figure 15). Compound 1 does not change to green or red because the accessible voids of compound 1 for rare earth ions are too small. Compound 3 exhibits the same phenomena as 1; the intensities of the characteristic fluorescence emission peaks of Eu3+ increase nonlinearly when the initial concentration of rare earth ion Eu3+ increases substantially (Figure 16a). As shown in Figure 16b, the luminescence intensity ratio of I (Eu3+, 616 nm) to I (3, 382 nm) increases and stabilizes at ∼6.5, which is much larger than the ratio of I (Eu3+) to I (1). The CIE coordinates also show that the luminescence color of 3 changes from blue to red when Eu3+ is adsorbed (Figure 17a). These

Figure 7. Adsorption isotherm of MB on 1 at ambient temperature.

absorption spectra (Figure 9), the maximum peak at 664 nm nearly disappeared after stirring for 120 min. As shown in Figure 10, a pseudo-second-order kinetic model can also be fitted to the adsorption kinetics of MB molecules on 3, with a smaller second-order kinetic constant of 1.09 × 10−3 g mg−1 min−1 (R2 = 0.997). The maximal adsorption capacity on 3 is also smaller than that on 1, which is ∼221.24 mg/g (Figure 11). Although compound 3 is not as good as 1, the secondorder kinetic constant of 3 is still higher than those of many reported materials applied to MB adsorption.30,38−41 With respect to MO adsorption, we obtain results similar to those for compound 1, as 3 also has the negatively charged framework. Under the same experimental conditions but with a 100 mg/L MO solution instead of a 13.09 mg/L MB solution, an only 2.30% decrease in the intensity of the signal was observed in the UV−vis absorption spectra at the maximum absorption peak of MO after stirring for 240 min, which indicates compound 3 shows almost no adsorption of MO molecules (Figure 9b). Theoretically, compound 3 has a solvent-accessible free volume that is larger than that of compound 1. However, the experimental results are the opposite. This may be related to the fact that the voids in compound 3 are distributed in all directions, some of which are irregular and narrow. The narrow channels are not conducive to the ingress and egress of solvent molecules and adsorption, which limits the application of 3 in dye adsorption. In addition, the functional groups could also play an important role.45−47 The functional -OCH3 groups in 1 enhance the interaction between the MB molecules and the MOFs, so that compound 1 has a larger second-order kinetic constant and a better adsorption capacity compared to those of 3. Selective Dye Adsorption on Compound 1 in an Aqueous Solution. The previous experiments have shown that compounds 1 and 3 might have selectively adsorb MB over MO. The better adsorbing material 1 was further studied

Figure 8. Photographs taken during the adsorption on 3 show the change in color of the MB solutions. 8401


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Figure 9. Sequential UV−vis spectra of (a) MB (13.09 mg/L) and (b) MO (100 mg/L) in an aqueous solution after the addition of 3.

Figure 10. (a) Adsorption kinetics of MB (13.09 mg/L) on compound 3 at ambient temperature. (b) Kinetic data fitting of MB (13.09 mg/L) adsorption on 3 using the pseudo-second-order kinetic model (R2 = 0.997).

Figure 11. Adsorption isotherm of MB on 3 at ambient temperature.

Figure 13. Sequential UV−vis spectra of the mixture of MB and MO in aqueous solution after the addition of 1.

data suggest that compound 3 can accommodate more Eu3+ ions than compound 1, which could be attributed to its larger theoretical solvent-accessible free volume (Figure 16c). Interestingly, the luminescence intensity ratio of I (Tb3+, 545 nm) to I (3, 382 nm) is just 0.47 when the initial concentration of Tb3+ ion is 1 × 10−2 mol/L. The normalized emission spectra of 3 (Tb3+) also imply that only a few Tb3+

ions are adsorbed by compound 3. This could be explained by the fact that the channels in 3 are too slim to contain the larger hydrated Tb3+ ions. Tuning the Chromaticity of Compound 1. The concentration of Eu3+ was kept at 1 × 10−3 mol/L, and the concentration of Tb3+ was modulated from 2 × 10−3 to 1 ×

Figure 12. Photographs taken during the adsorption on 1 show the change in color of the mixtures of MB and MO. 8402


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Figure 14. Normalized emission spectra of a suspension of 1 with an increasing initial concentration of a solution of (a) Eu3+ and (c) Tb3+. The luminescence intensity ratios of (b) I (Eu3+, 616 nm) to I (1, 421 nm) and (d) I (Tb3+, 545 nm) to I (1, 421 nm) plotted against the initial concentration of the corresponding rare earth ion show that compound 1 has similar Eu3+ and Tb3+ adsorption capacities.

Figure 15. CIE coordinates for 1 with different initial concentrations (1 × 10−7, 1 × 10−6, 1 × 10−5, 1 × 10−4, and 1 × 10−3 mol/L) of (a) Eu3+ and (b) Tb3+.

10−2 mol/L. The ground compound 1 was added to the solutions separately. The suspension was centrifuged and washed three times with distilled water. Then, the fluorescence of the suspension was measured. A tunable fluorescent material, which has near-white-light emission, was obtained. The CIE coordinates of the dots in Figure 17b are 1 (0.2761 0.2153), 2 (0.2686 0.2417), 3 (0.2645 0.2627), and 4 (0.2621 0.2833). Selective Rare Earth Ion Adsorption on Compound 3. The concentration of Tb3+ was kept at 1 × 10−3 mol/L, and the concentration of Eu3+ was modulated from 1 × 10−6 to 1 × 10−3 mol/L. The ground compound 3 was added to these solutions separately. The suspension was centrifuged and washed three times with distilled water. Then, the fluorescence of the suspension was measured. The normalized emission spectra shown in Figure 18a demonstrated that the intensity of the peak at 545 nm, which belongs to the characteristic

emission peaks of Tb3+, was decreasing while the intensity if the peak at 616 nm that belongs to the characteristic emission peaks of Eu3+ was observably increasing along with the increase in the Eu3+ concentration. The luminescence intensity ratio of I (Eu3+) to I (Eu3+ + Tb3+) was also remarkably increased when plotted against the initial concentration of Eu3+ (Figure 18b). It indicates that Eu3+ accounts for 71.9% of the total adsorption when the concentration of Eu3+ is one-hundredth of the concentration of Tb3+. The percentage increases to 92.1% when the concentration of Eu3+ is one-tenth of the concentration of Tb3+. Similar results had been obtained by increasing 10-fold the initial concentration of both Tb3+ and Eu3+ (Figure S9). It proved that compound 3 has the potential ability to selectively adsorb and identify rare earth ions Eu3+ and Tb3+. Compound 3 can be used for the removal of a small amount of Eu3+ from Tb3+ and the enrichment and purification of Eu3+. 8403


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Figure 16. Normalized emission spectra of a suspension of 3 with the increasing initial concentration of a solution of (a) Eu3+ and (c) Tb3+. (b) The luminescence intensity ratio of I (Eu3+, 616 nm) to I (3, 382 nm) plotted against the initial concentration of Eu3+ shows that compound 3 has a high Eu3+ adsorption capacity.

Figure 17. (a) CIE coordinates for 3 with different initial concentrations (1 × 10−7, 1 × 10−6, 1 × 10−5, 1 × 10−4, 1 × 10−3, and 1 × 10−2 mol/L) of Eu3+. (b) Tuning the chromaticity of 1 with a Tb3+/Eu3+ doping mixture (2:1, 5:1, 8:1, and 10:1), resulting in near-white-light emission.

Figure 18. (a) Emission spectra of a suspension of 3 with the increasing initial concentration of Eu3+ and a constant concentration of Tb3+ (1 × 10 −3 mol/L). (b) Luminescence intensity ratio of I (Eu3+) to I (Eu3+ + Tb3+) plotted against the concentration ratio of Eu3+ to Tb3+ at a Tb3+ concentration of 1 × 10 −3 mol/L.

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washed three times with DMF. The yield of the reaction was 50% [based on H4(G-1)]. Anal. Calcd for C71H99N10O21In: C, 55.25; H, 6.46; N, 9.08. Found: C, 55.26; H, 6.50; N, 9.07. The IR spectra of ligand H4(G-1) and compound 1 are shown in Figure S1a. A mixture of InCl3 (5 mg) and H4(G-2) (5 mg) was dissolved in 2 mL of DMF. After being subjected to ultrasonic treatment, the mixture was sealed in a 25 mL Parr Teflon-lined stainless steel autoclave under autogenous pressure and heated at 140 °C for 3 days. After the sample had cooled to ambient temperature, blocky yellow crystals were collected. The yield of the reaction was 35% [based on H4(G-2)]. However, when the solvent was removed, blocky crystals were crushed into powder. Therefore, the crystals were stored in DMF to maintain their integrity. Anal. Calcd for C87H137N18O24In: C, 54.03; H, 7.14; N, 13.04. Found: C, 54.05; H, 7.09; N, 13.03. The IR spectra of ligand H4(G-2) and compound 2 are shown in Figure S1b. A mixture of InCl3 (5 mg) and H4(G-3) (5 mg) was dissolved in 2 mL of DMF. Twenty microliters of CF3COOH was subsequently added. After being subjected to ultrasonic treatment, the mixture was sealed in a 25 mL Parr Teflon-lined stainless steel autoclave under autogenous pressure and heated at 140 °C for 3 days. After the sample had cooled to ambient temperature, dendritic colorless crystals were collected and washed three times with DMF. The yield of the reaction was 70% [based on H4(G-3)]. Anal. Calcd for C132H172N18O34In2: C, 56.94; H, 6.23; N, 9.05. Found: C, 56.92; H, 6.16; N, 9.04. The IR spectra of ligand H4(G-3) and compound 3 are shown in Figure S1c.

CONCLUSION In conclusion, three indium−organic frameworks have been successfully prepared through solvothermal reaction. The macroscopic morphology and the structures of these three compounds are different because of the different substituents in the ligands. Compounds 1 and 3 can adsorb cationic MB with good capacity and a high adsorption rate due to their anionic frameworks and channel-type voids. Moreover, compound 1 displays great selectivity of adsorbing cationic MB in the mixtures of MB and MO in aqueous solutions. The adsorption of rare earth ions (Eu3+ and Tb3+) of compounds 1 and 3 has been studied further. They show different adsorption characteristics of Eu3+ and Tb3+ due to their different structures and channel sizes caused by the different substituents of the ligands.

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EXPERIMENTAL SECTION

Materials and Measurement. All of the reagents and solvents were commercially available and used without further purification. The IR absorption spectra were recorded in the range of 400−4000 cm−1 on a Nicolet Impact 410 infrared spectrometer by using KBr pellets. SCXRD was measured by a Bruker Smart APEX CCD diffractometer. PXRD measurements were performed on a Bruker D8 Advance X-ray diffractometer, in which the Cu tube was operated at 40 kV and 40 mA, and used graphite monochromatic Mo Kα radiation (λ is 0.71073 Å) as the photosource. Thermogravimetric analyses (TGA) were performed with a Pyris 1 PerkinElmer thermogravimetric analyzer with a heating rate of 10 °C min−1 under a N2 atmosphere upon heating from room temperature to 800 °C. The UV−vis absorption spectra were recorded at room temperature by using a Shimadzu UV-3600 double monochromator spectrophotometer, and H2O was used as a reference solution. The fluorescence spectra were recorded at room temperature by using a HORIBA fluorescence spectrofluorometer. X-ray Crystallography. X-ray crystallographic data of compounds 1−3 were collected at room temperature by sealing a better crystal for SCXRD in a quartz tube with mother liquor on a Bruker Smart APEX CCD diffractometer that used an ω-scan and graphite monochromatic Mo Kα radiation (λ = 0.71073 Å) as the photosource. The intensity data were integrated by using SAINT. An empirical absorption correction was applied by using SADABS. The structures were determined by SHELXT, and the non-hydrogen atoms were located from the trial structures and then refined anisotropically with SHELXL-2018 using full-matrix least-squares procedures based on F2 values.48,49 The positions of the non-hydrogen atoms were refined with anisotropic displacement factors. The hydrogen atoms were positioned geometrically by using an idealized riding model. In the Check-cif report of 3, no level A alert was reported, but there were four unsolved level B alerts that were described as “low diffraction measured fraction theta full value”, “High wR2 value”, “Missing of FCF Reflection(s) below theta(Min)”, and “Low bond precision on C−C bonds”. These alerts were ascribed to poor single-crystal quality, large void space, and unsolved solvent molecules in the structure. Many crystals have been mounted with different methods (in a sealed capillary, in paratone oil, etc.), and the improvement in the quality of the data was limited. The crystal data presented in this work are the best we could achieve. The crystallographic data of compounds 1−3 are listed in Table S1. Selected bond lengths and angles are listed in Tables S2−S4. CCDC numbers 1897980−1897982 correspond to compounds 1−3, respectively. Synthesis of Compounds 1−3. A mixture of InCl3 (5 mg) and H4(G-1) (5 mg) was dissolved in 2 mL of DMF. Trace amounts of water and 30 μL of CF3COOH were added subsequently. After being subjected to ultrasonic treatment, the mixture was sealed in a 25 mL Parr Teflon-lined stainless steel autoclave under autogenous pressure and heated at 140 °C for 3 days. After the sample had cooled to ambient temperature, lamellar colorless crystals were collected and

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00499. IR spectra, PXRD patterns, TG diagrams, and other additional figures (PDF) Accession Codes

CCDC 1897980−1897982 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 86-25-89682309. ORCID

Hegen Zheng: 0000-0001-8763-9170 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by a grant from the National Natural Science Foundation of China (21771101). REFERENCES

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